A microphone is a transducer that converts sound into an electrical signal. Among different designs of microphone, a capacitive microphone or a condenser microphone is conventionally constructed employing the so-called “parallel-plate” capacitive design. Unlike other microphone types that require the sound wave to do more work, only a very small mass in capacitive microphones needs be moved by the incident sound wave. Capacitive microphones generally produce a high-quality audio signal and are now the popular choice in consumer electronics, laboratory and recording studio applications, ranging from telephone transmitters through inexpensive karaoke microphones to high-fidelity recording microphones.
FIG. 1 is a schematic diagram of parallel capacitive microphone in the prior art. Two thin layers 101 and 102 are placed closely in almost parallel. One of them is fixed backplate 101, and the other one is movable/deflectable membrane/diaphragm 102, which can be moved or driven by sound pressure. Diaphragm 102 acts as one plate of a capacitor, and the vibrations thereof produce changes in the distance between two layers 101 and 102, and changes in the mutual capacitance therebetween.
“Squeeze film” and “squeezed film” refer to a type of hydraulic or pneumatic damper for damping vibratory motion of a moving component with respect to a fixed component. Squeezed film damping occurs when the moving component is moving perpendicular and in close proximity to the surface of the fixed component (e.g., between approximately 2 and 50 micrometers). The squeezed film effect results from compressing and expanding the fluid (e.g., a gas or liquid) trapped in the space between the moving plate and the solid surface. The fluid has a high resistance, and damps the motion of the moving component as the fluid flows through the space between the moving plate and the solid surface.
In capacitive microphones as shown in FIG. 1, squeeze film damping occurs when two layers 101 and 102 are in close proximity to each other with air disposed between them. The layers 101 and 102 are positioned so close together (e.g. within 5 μm) that air can be “squeezed” and “stretched” to slow movement of membrane/diaphragm 101. As the gap between layers 101 and 102 shrinks, air must flow out of that region. The flow viscosity of air, therefore, gives rise to a force that resists the motion of moving membrane/diaphragm 101. Squeeze film damping is significant when membrane/diaphragm 101 has a large surface area to gap length ratio. Such squeeze film damping between the two layers 101 and 102 becomes a mechanical noise source, which is the dominating factor among all noise sources in the entire microphone structure.
Advantageously, the present invention provides a microphone design in which the squeeze film damping is substantially avoided because the movable membrane/diaphragm does not move into the fixed backplate.